1. Field of the Invention
This invention relates to VROM links and, more specifically, to a system and method for allowing the bi-directional flow of a current for programming the VROM link.
2. Background of the Invention
VROM links are made of an amorphous silicon sandwich (i.e., link). The VROM link has an opening when built which is used for programming the VROM link. At each end of the opening is an electrode. When programming the VROM link, a high voltage is applied across the two electrodes. The high voltage causes an antifuse material to break down at a certain weak point. As a programming current flows through the breakdown spot, it generates a large enough Joule heat to raise the temperature around the breakdown spot above the reaction temperature between the metal electrode and the antifuse layer. A conductive channel (i.e., filament) is formed through the reaction between the metal electrode and the antifuse material shorting one electrode to the other. The conductive channel is actually a core wherein the outside of the core is metal, and the inside of the core is a reaction product of metal and antifuse material. A detailed discussion of antifuses can be seen in "An Electro-Thermal Model For Metal-Oxide-Metal Antifuses," IEEE Transactions On Electron Devices, Vol. 42, No. 8, August 1995, pages 1548-1558.
The programming current is defined as the largest current flow through an antifuse which last at least 10 ns. If the duration of the programming current is shorter than 10 ns, a stable stage will not be reached and no stable conductive link will be formed. The formation geometry of the VROM link is related to the direction of the current flow during programming. In general, the diameter of the filament is fat on one electrode and thin on the opposite electrode. Thus, the programming current is limited to one direction only. Reversing the current during the programming event will have the effect of increasing the thin diameter of the filament. This will allow an increase in the programming yield of the VROM link. Furthermore, since some technologies will use programming voltages less than p-channel breakdown, level shifters can be used to steer the programming voltage instead of the strapped n-channels and charge pumps currently being used. This will allow simplification of the programming circuit.
Therefore, a need existed to provide a system and method for allowing bi-direction current flow during programming of the VROM link thereby increasing the programming yield of the VROM link. The system and method must also simplify the circuit used for programming the VROM link.